Porous film and method for producing same

ABSTRACT

Provided are a porous film having excellent surface smoothness and a method for producing the same. The surface roughness of a porous film of polyvinylidene fluoride, polyethersulfone, polyimide and/or polyamide-imide is Ra 30,000 Å or less. The opening diameter of the porous film is preferably from 100 nm to 5000 nm. The method for producing a porous film preferably includes a step for kneading a varnish containing fine particles and at least one resin selected from the group consisting of polyvinylidene fluoride, polyether sulfone, polyamic acid, polyimide, polyamide-imide precursor, and polyamide-imide. The varnish preferably has a viscosity at 25° C. of 0.1-3 Pa·s, a solids fraction concentration of 10-50 mass %, and a fine particle average particle size of 10-5000 nm.

TECHNICAL FIELD

The present invention relates to a porous film and a method forproducing the same.

BACKGROUND ART

Conventionally, various porous films have been used for a filter and thelike. Furthermore, in recent years, application of porous films toseparators for secondary batteries such as a lithium battery has beenprogressed.

Examples of the well-known method for producing a polyimide porous filminclude a method which includes applying a varnish obtained bydispersing silica particles in a solution of polyamide acid or polyimideonto a substrate to form a coating film, then heating the coating filmas necessary so as to obtain a polyimide film containing silicaparticles, and then removing silica in the polyimide film by elutionusing a hydrogen fluoride solution so as to make the film porous (seePatent Document 1).

Patent Document 1: Japanese Patent No. 5605566

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

As the above-mentioned porous film, a porous film having smooth surfaceis required. For example, in use of a separator for a secondary battery,by allowing a separator and an electrode to satisfactorily adhere toeach other, the battery performance is improved. When a surface of aporous film to be used as a separator is smooth, satisfactory adhesionbetween the separator and the electrode is achieved. Furthermore, in afilter as a film for separation of gas or liquid, a porous film and asupport for supporting the porous film are desired to satisfactorilyadhere to each other. When the porous film and the supportsatisfactorily adhere to each other, separation performance and handlingproperty of a filter are improved. When a surface of a porous film issmooth, the permeation property (liquid permeation property) of theporous film is improved and handling also becomes good.

The present invention has been made in view of the above-mentionedcircumstances, and an object of the present invention is to provide aporous film having excellent surface smoothness and a method forproducing the porous film.

Means for Solving the Problems

In order to solve the above-mentioned problems, the present inventorshave conducted extensive studies. As a result, they have found that aporous film having excellent surface smoothness can be obtained byadjusting conditions such as kneading time in the production conditionsof a varnish to be used for producing a porous film, and they havecompleted the present invention.

A first embodiment of the present invention relates to a porous film ofpolyvinylidene fluoride, polyethersulfone, polyimide and/orpolyamide-imide, wherein the surface roughness Ra is 30000 Å or less.

A second embodiment of the present invention relates to a porous film ofpolyvinylidene fluoride, polyethersulfone; polyimide and/orpolyamide-imide, wherein the porous film includes a structure in whichspherical pores communicate with each other, no waviness is deeper thana recess formed by an opening portion, and a diameter of the openingportion is 100 nm to 5000 nm.

A third embodiment of the present invention relates to a method forproducing a porous film of polyvinylidene fluoride, polyimide and/orpolyamide-imide, the method including a step of kneading a varnishcontaining fine particles and a resin such that surface roughness Ra ofan unburned composite film containing the fine particles, and at leastone resin selected from the group consisting of polyvinylidene fluoride,polyethersulfone, polyamide acid, polyimide, a polyamide-imide precursorand polyamide-imide is 30000 Å or less.

A fourth embodiment of the present invention relates to a method forproducing a porous film of polyvinylidene fluoride, polyethersulfone,polyimide and/or polyamide-imide, the method comprising a step ofkneading a varnish containing fine particles, and at least one resinselected from the group consisting of polyvinylidene fluoride,polyethersulfone, polyamide acid, polyimide, a polyamide-imide precursorand polyamide-imide, wherein the varnish has a viscosity at 25° C. of0.1 to 3 Pa·s, the solid content concentration of 10 to 50% by mass, andthe average particle diameter of the fine particles of 10 to 5000 nm,and the kneading is carried out for 2 minutes to 10 hours.

Effects of the Invention

The present invention can provide a porous film having excellent surfacesmoothness and a method for producing the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an electron microscope image of a porous film obtained inExample 1.

FIG. 2 shows an electron microscope image of a porous film obtained inComparative Example 1.

PREFERRED MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail, but the present invention is not necessarily limited to thefollowing embodiments and can be implemented with appropriatemodifications within the purpose of the present invention.

<<Porous Film of Polyvinylidene Fluoride, Polyethersulfone, Polyimideand/or Polyamide-Imide>>

A porous film includes polyvinylidene fluoride, polyethersulfone,polyimide and/or polyamide-imide. The surface roughness Ra of the porousfilm is 30000 Å or less, preferably 10000 Å or less, more preferably5000 Å or less, and further preferably 3500 Å or less. Therefore, theporous film has excellent smoothness.

When the surface roughness Ra of the porous film is more than 30000 Å,convection occurs in an unburned composite film including fine particlesand a resin. Accordingly, the value of surface roughness of the unburnedcomposite film is increased in many cases. When the convection occurs inthe unburned composite film, distribution of fine particles in theunburned composite film tends to be inconsistent. For example, in theunburned composite film, fine particles are dense in a portion that is avalley portion having smaller thickness while fine particles arenon-dense in a portion that is a mountain portion having largerthickness; fine particles are non-dense in a mountain portion while fineparticles are dense in a valley portion. In this case, as a result ofremoving fine particles from the unburned composite film to form aporous film, a porous film having variation in distribution of openingsis obtained. On the contrary, when the surface roughness of the porousfilm is 30000 Å or less, convection does not occur or only mildconvection occurs in many cases in an unburned composite film includingfine particles and a resin. As a result, in a case where the surfaceroughness of a porous film is 30000 Å or less, a porous film in whichdistribution of openings in the surface is uniform is easily obtained.

Furthermore, when the surface roughness Ra is in the above-mentionedrange, when the porous film is used as a separator of a battery, theporous film satisfactorily adheres to an electrode. Furthermore, thesurface roughness Ra in the above-mentioned range improves permeationproperty of the porous film when the porous film is used as a filter.When the porous film as a filter adheres to a support, peeling of thesupport of the filter is suppressed when filtering is carried out, andaccordingly rupture in the filter and other breakages are alsosuppressed, and the handling property is improved.

In claims and specification of the present application, the surfaceroughness Ra is a value measured using a stylus type surface roughnessmeter according to the following conditions. As the stylus type surfaceroughness meter, Dektak150 manufactured by Ulvac Inc. can be used.Stylus

radius: 12.5 μmMeasurement distance: 10000 μmMeasurement time: 120 secondsHorizontal resolution: 0.278 μm/sampleStylus pressure: 5.00 mg

Furthermore, the porous film includes a structure in which sphericalpores communicate with each other (hereinafter, abbreviated as a“continuous pore”). Note here that this porous film is produced as afilm formed on a substrate as mentioned later. The surface of the porousfilm is a surface opposite to the surface facing the substrate at thetime of production. An opening portion in the porous film is referred toas a portion in which the above-mentioned continuous pore is opened onthe surface of the porous film. Preferably, the porous film does nothave waviness in the surface thereof. The waviness is referred to as “arecess portion that is deeper than a recess formed by an openingportion” in the surface in which the surface roughness is measured. Therecess formed by the opening portion corresponds to a spherical poreformed in the surface of a film when the porous film is produced. Arecess portion that is deeper than the depth from the film surface ofthe spherical pore corresponds to the waviness.

The spherical shape with respect to a pore shape includes atrue-spherical shape, but it is not necessarily limited to atrue-spherical shape. The spherical shape is only required to be asubstantially true-spherical shape, and shapes that can be recognized tobe substantially a true-spherical shape when an enlarged image of a porepart is visually observed is also included in spherical shapes.Specifically, in a spherical pore, a surface that defines a pore part isa curved surface, and a true-spherical shape or substantially atrue-spherical shape are only required to be defined by the curvedsurface.

Typically, individual spherical pores are formed in the post step byremoving individual fine particles existing in the below-mentionedresin-fine particle composite film. The continuous pore is formed in thepost step by removing a plurality of fine particles that are present incontact with each other in resin-fine particle composite film in thebelow-mentioned method for producing a porous film. A section in whichspherical pores communicate with each other in the continuous pore isderived from a section in which a plurality of fine particles beforebeing removed are brought into contact with each other.

A diameter of the opening portion in the porous film may beappropriately varied in range of, for example, 100 nm to 5000 nmdepending on the applications of use of the porous film. In separatorapplications, the diameter is preferably 100 to 2000 nm, more preferably200 to 1000 nm, and further preferably 300 to 900 nm. The diameter ofthe opening portion is equal or substantially equal to the diameter of aspherical pore constituting the continuous pore. The continuous poreformed by linking spherical pores each having such a diametersatisfactorily allows fluid to pass through the porous film. The porousfilm has a continuous pore inside thereof as a fluid flow passagepenetrating in a thickness direction of the porous film. This enables afluid to penetrate from one main surface to the other main surface ofthe porous film. Furthermore, when a laminated body is used as a filter,a fluid passes through the inside of a porous film while it is broughtinto contact with curved surfaces defining individual spherical pores.The contact area of the fluid inside the porous film is relatively largebecause a continuous pore of spherical pores is provided. Therefore,when a fluid is allowed to pass through a laminated body including aporous film, it is considered that minute substances that are present inthe fluid are easily adsorbed to spherical pores in the porous film.

<<Varnish for Producing Porous Film>>

For production of the porous film mentioned above, a varnish forproducing a porous film (hereinafter, which may be simply referred to asa “varnish”) is used. The varnish contains predetermined fine particles,a resin, and a solvent, and the resin is dissolved in a solvent. Thevarnish is typically produced by a fine particle dispersion liquidpreparation step of dispersing fine particles into a solvent; a step ofpreparing a resin solution containing at least one resin selected fromthe group consisting of polyvinylidene fluoride, polyethersulfone,polyamide acid, polyimide, polyamide acid that is to be apolyamide-imide precursor, and polyamide-imide, and a kneading step ofkneading the fine particle dispersion liquid and the resin solutiontogether so as to adjust the concentration.

In the varnish, it is preferable that a viscosity at 25° C. is 0.1 to 3Pa·s or more, a solid content concentration is 10 to 50% by mass, and anaverage particle diameter of fine particles is 10 to 5000 nm. Thevarnish is preferably produced by kneading for, preferably two minutesto ten hours, and more preferably two minutes to 60 minutes. Note herethat the viscosity of the varnish is measured using an E-typeviscometer. The varnish can be kneaded using a rotation-revolution typemixer (product name: “Awatori Rentarou” manufactured by ThinkyCorporation), a planetary mixer, a beads mill, and the like. Theviscosity of the varnish for producing a porous film and the solidcontent concentration are adjusted to the above-mentioned range, aporous film having a desired smoothness can be easily formed. When thevarnish is kneaded for an appropriate range of time, a smooth porousfilm can be easily formed using the varnish. When the kneading time isnot less than two minutes, a porous film formed using the varnish caneasily achieve the desired smoothness having surface roughness Ra of30000 Å or less. From the viewpoint of the throughput, the kneading timeof the varnish is preferably not beyond 10 hours. Furthermore, use offine particles having the above-mentioned range of average particlediameter enables an opening portion having a desirable size to be formedin a surface of a porous film, and enables a continuous pore in whichspherical pores having a desirable size continue to be formed inside theporous film.

[Resin]

As mentioned above, the varnish includes at least one resin selectedfrom the group consisting of polyvinylidene fluoride, polyethersulfone,polyamide acid, polyimide, a polyamide-imide precursor andpolyamide-imide. Hereinafter, these resins will be described.

[Polyvinylidene Fluoride]

Polyvinylidene fluoride is not particularly limited as long as it can bedissolved in a solvent to be used for forming a varnish. Thepolyvinylidene fluoride may be a homopolymer or may be a copolymer(copolymerized product). Examples of constituent units to becopolymerized include ethylene, polychlorotrifluoroethylene,tetrafluoroethylene, hexafluoropropylene, or the like. The mass averagemolecular weight thereof is, for example, about 10,000 to 5,000,000.

[Polyethersulfone]

Polyethersulfone is not particularly limited as long as it can bedissolved in a solvent to be used for forming a varnish. Thepolyethersulfone can be appropriately selected depending on theapplications of use of porous films to be produced, and it may behydrophilic or may be hydrophobic. Furthermore, the polyethersulfone maybe aliphatic polyethersulfone or may be aromatic polyethersulfone. Themass average molecular weight thereof is, for example, 5000 to1,000,000, and preferably 10,000 to 300,000.

[Polyamide Acid]

The polyamide acid may be any one prepared by polymerizing appropriatetetracarboxylic dianhydride and diamine. The amounts of thetetracarboxylic dianhydride and the diamine to be used are notparticularly limited, and the amount of the diamine is preferably 0.50to 1.50 mol, more preferably 0.60 to 1.30 mol, and particularlypreferably 0.70 to 1.20 mol, based on 1 mol of the tetracarboxylicdianhydride.

The tetracarboxylic dianhydride can be appropriately selected fromtetracarboxylic dianhydrides that have been conventionally used as rawmaterials for synthesizing polyamide acids. The tetracarboxylicdianhydride may be an aromatic tetracarboxylic dianhydride or analiphatic tetracarboxylic dianhydride, but from the viewpoint of theheat resistance of the resulting polyimide resin, an aromatictetracarboxylic dianhydride is preferably used. One type oftetracarboxylic dianhydride may be used alone or may be used in acombination of two or more types thereof.

Preferred examples of the aromatic tetracarboxylic dianhydride includepyromellitic dianhydride, 1,1-bis(2,3-dicarboxyphenyl) ethanedianhydride, bis(2,3-dicarboxyphenyl)methane dianhydride,bis(3,4-dicarboxyphenyl)methane dianhydride,3,3′,4,4′-biphenyltetracarboxylic dianhydride,2,3,3′,4′-biphenyltetracarboxylic dianhydride,2,2,6,6-biphenyltetracarboxylic dianhydride,2,2-bis(3,4-dicarboxyphenyl)propane dianhydride,2,2-bis(2,3-dicarboxyphenyl)propane dianhydride,2,2-bis(3,4-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride,2,2-bis(2,3-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride,3,3′,4,4′-benzophenonetetracarboxylic dianhydride,bis(3,4-dicarboxyphenyl)ether dianhydride, bis(2,3-dicarboxyphenyl)etherdianhydride, 2,2′,3,3′-benzophenonetetracarboxylic dianhydride,4,4-(p-phenylenedioxy)diphthalic dianhydride,4,4-(m-phenylenedioxy)diphthalic dianhydride,1,2,5,6-naphthalenetetracarboxylic dianhydride,1,4,5,8-naphthalenetetracarboxylic dianhydride,2,3,6,7-naphthalenetetracarboxylic dianhydride,1,2,3,4-benzenetetracarboxylic dianhydride,3,4,9,10-perylenetetracarboxylic dianhydride,2,3,6,7-anthracenetetracarboxylic dianhydride,1,2,7,8-phenanthrenetetracarboxylic dianhydride, 9,9-bisphthalicanhydride fluorene, and 3,3′,4,4′-diphenylsulfonetetracarboxylicdianhydride. Examples of the aliphatic tetracarboxylic dianhydrideinclude ethylenetetracarboxylic dianhydride, butanetetracarboxylicdianhydride, cyclopentanetetracarboxylic dianhydride,cyclohexanetetracarboxylic dianhydride,1,2,4,5-cyclohexanetetracarboxylic dianhydride, and1,2,3,4-cyclohexanetetracarboxylic dianhydride. Among them,3,3′,4,4′-biphenyltetracarboxylic dianhydride and pyromelliticdianhydride are preferred because of their inexpensiveness and readyavailability. One type of tetracarboxylic dianhydride may be used aloneor as a mixture of two or more types thereof.

The diamine can be appropriately selected from diamines that have beenconventionally used as raw materials for synthesizing polyamide acids.The diamine may be an aromatic diamine or an aliphatic diamine, but fromthe viewpoint of the heat resistance of the resulting polyimide resin,an aromatic diamine is preferred. One type of these diamine may be usedalone or in a combination of two or more types thereof.

Examples of the aromatic diamine include diamino compounds having onephenyl group or about two to ten phenyl groups. Specifically, examplesof the aromatic diamine include phenylenediamines and their derivatives,diaminobiphenyl compounds and their derivatives, diaminodiphenylcompounds and their derivatives, diaminotriphenyl compounds and theirderivatives, diaminonaphthalenes and their derivatives,aminophenylaminoindanes and their derivatives, diaminotetraphenylcompounds and their derivatives, diaminohexaphenyl compounds and theirderivatives, and cardo-type fluorenediamine derivatives.

The phenylenediamines are, for example, m-phenylenediamine andp-phenylenediamine. The phenylenediamine derivatives are diamines towhich alkyl groups, such as a methyl group or an ethyl group, are bound,such as 2,4-diaminotoluene and 2,4-triphenylenediamine.

In the diaminodiphenyl compounds, two aminophenyl groups are bonded toeach other. For example, the diaminodiphenyl compounds are4,4′-diaminobiphenyl and 4,4′-diamino-2,2′-bis(trifluoromethyl)biphenyl.

The diaminodiphenyl compound is a compound obtained by linkage of twoaminophenyl groups at their phenyl groups via another group. The linkageis, for example, an ether linkage, a sulfonyl linkage, a thioetherlinkage, a linkage of an alkylene or its derivative group, an iminolinkage, an azo linkage, a phosphine oxide linkage, an amide linkage, oran ureylene linkage. The number of carbon atoms of the alkylene linkageis about 1 to 6. The derivative groups is an alkylene group whose one ormore hydrogen atoms have been replaced by, for example, halogen atoms.

Examples of the diaminodiphenyl compounds include 3,3′-diaminodiphenylether, 3,4′-diaminodiphenyl ether, 4,4′-diaminodiphenyl ether,3,3′-diaminodiphenyl sulfone, 3,4′-diaminodiphenyl sulfone,4,4′-diaminodiphenyl sulfone, 3,3′-diaminodiphenyl methane,3,4′-diaminodiphenyl methane, 4,4′-diaminodiphenyl methane,4,4′-diaminodiphenyl sulfide, 3,3′-diaminodiphenyl ketone,3,4′-diaminodiphenyl ketone, 2,2-bis(p-aminophenyl)propane,2,2′-bis(p-aminophenyl)hexafluoropropane,4-methyl-2,4-bis(p-aminophenyl)-1-pentene,4-methyl-2,4-bis(p-aminophenyl)-2-pentene, iminodianiline,4-methyl-2,4-bis(p-aminophenyl)pentane, bis(p-aminophenyl)phosphineoxide, 4,4′-diaminoazobenzene, 4,4′-diaminodiphenylurea,4,4′-diaminodiphenylamide, 1,4-bis(4-aminophenoxy)benzene,1,3-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene,4,4′-bis(4-aminophenoxy)biphenyl, bis[4-(4-aminophenoxy)phenyl] sulfone,bis[4-(3-aminophenoxy)phenyl] sulfone, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, and 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane.

Among these, p-phenylenediamine, m-phenylenediamine, 2,4-diaminotoluene,and 4,4′-diaminodiphenylether are preferred because of theirinexpensiveness and ready availability.

The diaminotriphenyl compound is formed by linkage of two aminophenylgroups and one phenylene group, all of which are each linked throughanother group. The “another group” is selected from the same groups asin the diaminodiphenyl compounds. Examples of the diaminotriphenylcompounds include 1,3-bis(m-aminophenoxy)benzene,1,3-bis(p-aminophenoxy)benzene, and 1,4-bis(p-aminophenoxy)benzene.

Examples of the diaminonaphthalenes include 1,5-diaminonaphthalene and2,6-diaminonaphthalene.

Examples of the aminophenylaminoindanes include 5- or6-amino-1-(p-aminophenyl)-1,3,3-trimethylindane.

Examples of the diaminotetraphenyl compounds include4,4′-bis(p-aminophenoxy)biphenyl,2,2′-bis[p-(p′-aminophenoxy)phenyl]propane,2,2′-bis[p-(p′-aminophenoxy)biphenyl]propane, and2,2′-bis[p-(m-aminophenoxy)phenyl]benzophenone.

An example of the cardo-type fluorenediamine derivatives is9,9-bisanilinefluorene.

The number of carbon atoms of aliphatic diamine is, for example, about 2to 15. Specific examples of aliphatic diamine includepentamethylenediamine, hexamethylenediamine, and heptamethylenediamine.

Note here that hydrogen atoms of these diamines may be a compound havingat least one substituent selected from the group consisting of halogenatoms and methyl, methoxy, cyano, and phenyl groups.

There is no particular limitation to means for producing the polyamideacid, and, for example, well-known technique such as a method forreacting an acid and a diamine component in a solvent can be used.

The reaction of a tetracarboxylic dianhydride and a diamine is usuallyperformed in a solvent. The solvent to be used for the reaction of atetracarboxylic dianhydride and a diamine is not particularly limitedand may be any solvents that can dissolve the tetracarboxylicdianhydride and the diamine without reacting with the tetracarboxylicdianhydride and the diamine. One type of solvent may be used alone or ina combination of two or more types thereof.

Examples of the solvent to be used for the reaction of a tetracarboxylicdianhydride and a diamine include nitrogen-containing polar solvents,such as N-methyl-2-pyrrolidone, N,N-dimethyl acetamide,N,N-diethylacetamide, N,N-dimethylformamide, N,N-diethylformamide,N-methylcaprolactam, and N,N,N′,N′-tetramethylurea; lactone polarsolvents, such as (i-propiolactone, γ-butyrolactone, γ-valerolactone,δ-valerolactone, γ-caprolactone, and ε-caprolactone; dimethyl sulfoxide;acetonitrile; fatty acid esters, such as ethyl lactate and butyllactate; ethers, such as diethylene glycol dimethyl ether, diethyleneglycol diethyl ether, dioxane, tetrahydrofuran, methyl cellosolveacetate, and ethyl cellosolve acetate; and phenol solvents, such ascresols and xylene-based mixed solvent. One type of these solvents maybe used alone or in a combination of two or more types thereof. Theamount of the solvent to be used is not particularly limited but isdesirably an amount such that the content of the resulting polyamideacid is 5% to 50% by mass.

Among these solvents, from the viewpoint of the solubility of theresulting polyamide acid, preferred are nitrogen-containing polarsolvents, such as N-methyl-2-pyrrolidone, N,N-dimethyl acetamide,N,N-diethylacetamide, N,N-dimethylformamide, N,N-diethylformamide,N-methylcaprolactam, and N,N,N′,N′-tetramethylurea.

The polymerization temperature is usually −10° C. to 120° C. andpreferably 5° C. to 30° C. The polymerization time varies depending onthe raw material composition to be used, and is usually 3 to 24 Hr(hours). One type of polyamide acid may be used alone or in acombination of two or more types thereof.

[Polyimide]

The polyimide can be any known polyimide, without any limitation to itsstructure and molecular weight. The side chain of the polyimide may havea condensable functional group, such as a carboxy group, or a functionalgroup enhancing the cross-linking reaction during burning. Furthermore,when the varnish contains a solvent, the soluble polyimide that can bedissolved in a solvent used is preferable.

In order to make the polyimide soluble in a solvent, it is effective touse a monomer for introducing a flexible bend structure into the mainchain, for example, to use an aliphatic diamine, such asethylenediamine, hexamethylenediamine, 1,4-diaminocyclohexane,1,3-diaminocyclohexane, or 4,4′-diaminodicyclohexylmethane; an aromaticdiamine, such as 2-methyl-1,4-phenylenediamine, o-tolidine, m-tolidine,3,3′-dimethoxybenzidine, or 4,4′-diaminobenzanilide; apolyoxyalkylenediamine, such as polyoxyethylenediamine,polyoxypropylenediamine, or polyoxybutyrenediamine; apolysiloxanediamine; 2,3,3′,4′-oxydiphthalic anhydride,3,4,3′,4′-oxydiphthalic anhydride, or2,2-bis(4-hydroxyphenyl)propanedibenzoate-3,3′,4,4′-tetracarboxylicdianhydride. It is also effective to use a monomer containing afunctional group for improving the solubility in a solvent, for example,to use a fluorinated diamine, such as2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl or2-trifluoromethyl-1,4-phenylenediamine. Furthermore, in addition to themonomer for improving the solubility of the polyimide, a monomer that ismentioned in the paragraph describing the polyamide acid may be usedwithin a range that does not inhibit the solubility. For each ofpolyimide and the monomer thereof, one type thereof may be used alone orin a combination of two or more types thereof.

There is no limitation to a method for producing polyimide. Polyimidemay be produced by any well-known techniques, for example, chemicallyimidizing or thermally imidizing polyamide acid. Examples of suchpolyimides include aliphatic polyimide (full-aliphatic polyimides) andaromatic polyimides, and aromatic polyimides are preferable. Thearomatic polyimide may be one prepared by a thermal or chemicalring-closing reaction of a polyamide acid having repeating unitsrepresented by Formula (1) or a polyimide having repeating unitsrepresented by Formula (2). In the formulae, Ar represents an arylgroup. When the varnish contains a solvent, these polyimides may be thendissolved in a solvent to be used.

[Polyamide-Imide and Polyamide-Imide Precursor]

Any well-known polyamide-imides can be used without limitation to thestructure or molecular weight. The side chain of the polyamide-imide mayhave a condensable functional group, such as a carboxy group, or afunctional group enhancing the cross-linking reaction during burning.Furthermore, when the varnish contains a solvent, a solublepolyamide-imide that can be dissolved in a solvent to be used ispreferable.

As the polyamide-imide, (i) a resin obtained by reacting an acid havinga carboxyl group and an acid anhydride group with diisocyanate in onemolecule of trimellitic anhydride and the like, (ii) a resin obtained byimidization of a precursor polymer obtained by reacting a reactivederivative of the acid such as trimellitic anhydride chloride anddiamine (a polyamide-imide precursor) can be usually used withoutparticular limitation.

Examples of the above-mentioned acids or the reactive derivativesinclude trimellitic anhydride, trimellitic anhydride halides such astrimellitic anhydride chloride, trimellitic anhydride esters, and thelike.

Examples of the above-mentioned optional diamine include diaminesdescribed as an example in the description of the above-mentionedpolyamide acid. A diaminopyridine compound can also be used.

The above mentioned any diisocyanate is not particularly limited, andincludes, for example, a diisocyanate compound corresponding to theabove-mentioned optional diamine can be used. Specific examples thereofinclude meta-phenylene diisocyanate, p-phenylene diisocyanate,o-tolidine diisocyanate, p-phenylene diisocyanate, m-phenylenediisocyanate, 4,4′-oxybis (phenyl isocyanate), 4,4′-diphenylmethanediisocyanate, bis[4-(4-isocyanate phenoxy) phenyl] sulfone,2,2′-bis[4-(4-isocyanate phenoxy) phenyl] propane, 2,4-tolylenediisocyanate, 2,6-tolylene diisocyanate, 4,4′-diphenylmethanediisocyanate, 3,3′-dimethyldiphenyl-4,4′-diisocyanate,3,3′-diethyldiphenyl-4,4′-diisocyanate, isophorone diisocyanate,hexamethylene diisocyanate, 4,4′-dicyclohexyl methane diisocyanate,m-xylene diisocyanate, p-xylene diisocyanate, naphtalen diisocyanate,and the like, can be exemplified.

As a raw material monomer of polyamide-imide, in addition to the above,it is possible to use compounds described as general formulae inJapanese Unexamined Patent Application, Publication No. S63-283705 andJapanese Unexamined Patent Application, Publication No. H2-198619.Furthermore, the imidization in the method described in the above (ii)may be any one of thermal imidization and chemical imidization. As thechemical imidization, a method of immersing an unburned composite filmformed by using a varnish such as a polyamide-imide precursor in a mixedsolvent including acetic anhydride, or acetic anhydride andisoquinoline, and the like can be used. Note here that thepolyamide-imide precursor can be referred to as a polyimide precursorfrom the viewpoint that it is a precursor before imidization.

The polyamide-imide to be contained in a varnish may be theabove-mentioned (1) a polymer obtained by reacting an acid such astrimellitic anhydride and diisocyanate with each other, (2) a polymerobtained by imidization of a precursor polymer obtained by reacting areactive derivative of the above-mentioned acid such as trimelliticanhydride chloride and diamine with each other, or the like. The term“polyamide-imide precursor” in this specification and claims means apolymer before imidization (a precursor polymer). For each ofpolyamide-imide and polyamide-imide precursor, one type thereof may beused alone or in a combination of two or more types thereof.Furthermore, as the polyamide-imide, for each of the above-mentionedpolymer, raw material monomer, and oligomer, one type thereof may beused alone or in a combination of two or more types thereof.

[Fine Particles]

The material of the fine particles is not particularly limited and maybe any known material as long as the material is insoluble in thesolvent contained in the varnish and can be removed later fromresin-fine particle composite film. Examples of the inorganic materialinclude metal oxides, such as silica (silicon dioxide), titanium oxide,and alumina (Al₂O₃). Examples of the organic materials includehigh-molecular-weight olefins (such as polypropylene and polyethylene)and organic polymer fine particles, such as polystyrenes, epoxy resins,celluloses, polyvinyl alcohols, polyvinyl butyrals, polyesters, andpolyethers.

Specific examples of the fine particles include colloidal silica. Inparticular, it is preferable to select monodisperse spherical silicaparticles because uniform pores can be formed.

Furthermore, it is preferable that the fine particles preferably have ahigh sphericity and a low particle diameter distribution index. Fineparticles satisfying these conditions show excellent dispersibility inthe varnish and can be used without causing aggregation with oneanother. The average particle diameter of the fine particles ispreferably 100 to 5000 nm. When such conditions are satisfied, the porediameter of the porous film obtained by removing fine particles can beadjusted. In the case of a separator, fine particles having an averageparticle diameter of 100 to 2000 nm are preferably used, becauseelectric field applied to the obtained porous film can be made uniform.One type of fine particle may be used alone or in a combination of twoor more types thereof.

[Solvent]

Any solvents may be used as long as they can solve a resin whichincludes polyvinylidene fluoride, polyethersulfone, polyamide acidand/or polyimide and which does not solve fine particles. Examples ofthe solvent include solvents described as an example of a solvent to beused for reaction of tetracarboxylic dianhydride and diamine. One typeof solvent may be used alone or in a combination of two or more typesthereof. For polyvinylidene fluoride, examples of the solvent includelower alkylketone such as methyl ethyl ketone, acetone, andtetrahydrofuran, and trimethyl phosphate, and the like, in addition tothe above-mentioned nitrogen-containing polarity solvent. Forpolyethersulfone, examples of the solvent include polar solvents such asdiphenylsulfone, dimethylsulfone, dimethylsulfoxide, benzophenone,tetrahydrothiophene-1,1-dioxide, and 1,3-dimethyl-2-imidazolidinone, inaddition to the above-mentioned nitrogen-containing polarity solvent.

[Dispersant]

In order to uniformly disperse fine particles in a varnish, a dispersanttogether with fine particles may be added. Addition of the dispersantallows further uniform mixing of the fine particles in a varnish, andfurther allows uniform dispersion of the fine particles in a filmincluding a varnish. As a result, dense openings are provided on thesurface of the finally obtained porous film, and the front and rearsurfaces can be allowed to efficiently communicate with each other, thusimproving the air permeability of the porous film. Furthermore, additionof the dispersant easily improves drying of the varnish, and easilyimproves peelability of the formed unburned composite film from asubstrate and the like.

The dispersant is not particularly limited and any known dispersant maybe used. Examples of the dispersant include, but not limited to, anionicsurfactants, such as salts of coconut fatty acid, salts of sulfonatedcastor oil, lauryl sulfate, polyoxyalkylene allylphenyl ether sulfate,alkylbenzenesulfonic acid, alkylbenzene sulfonate, alkyldiphenyl etherdisulfonate, alkylnaphthalene sulfonate, dialkyl sulfosuccinate,isopropyl phosphate, polyoxyethylene alkyl ether phosphate, andpolyoxyethylene allylphenyl ether phosphate; cationic surfactants, suchas oleylamine acetate, lauryl pyridinium chloride, cetyl pyridiniumchloride, lauryl trimethylammonium chloride, stearyl trimethylammoniumchloride, behenyl trimethylammonium chloride, and didecyldimethylammonium chloride; amphoteric surfactants, such as coconut alkyldimethylamine oxide, fatty acid amide propyl dimethyl amine oxide, alkylpolyaminoethyl glycine hydrochloride, amide betaine surfactant, alaninesurfactant, and lauryl iminodipropionic acid; polyoxyalkylene primaryalkyl ether or polyoxyalkylene secondary alkyl ether nonionicsurfactants, such as polyoxyethylene octyl ether, polyoxyethylene decylether, polyoxyethylene lauryl ether, polyoxyethylene laurylamine,polyoxyethylene oleylamine, polyoxyethylene polystyryl phenyl ether, andpolyoxyalkylene polystyryl phenyl ether; other polyoxyalkylene nonionicsurfactants, such as polyoxyethylene dilaurate, polyoxyethylene laurate,polyoxyethylenated castor oil, polyoxyethylenated hydrogenated castoroil, sorbitan laurate, polyoxyethylene sorbitan laurate, and fatty aciddiethanolamide; fatty acid alkyl esters, such as octyl stearate andtrimethylolpropane tridecanoate; and polyether polyols, such aspolyoxyalkylene butyl ether, polyoxyalkylene oleyl ether, andtrimethylol propane tris(polyoxyalkylene) ether. These dispersants maybe used as a mixture of two or more types thereof.

From the view point of, for example, the film formability, the contentof the dispersant in the varnish is preferably 0.01 to 5% by mass, morepreferably 0.05 to 1% by mass, and further more preferably 0.1 to 0.5%by mass, with respect to the mass of the fine particles.

<<Method for Producing Porous Film>>

A typical method for producing a porous film includes an unburnedcomposite film forming step of forming an unburned composite film usinga varnish; a burning step of burning the unburned composite film toobtain a resin-fine particle composite film; and a fineparticle-removing step of removing the fine particles from theresin-fine particle composite film.

<Method for Producing Unburned Composite Film (Unburned Composite FilmForming Step)>

Hereinafter, a method for forming an unburned composite film will bedescribed. In an unburned composite film forming step, an unburnedcomposite film is formed using a varnish. At this time, the unburnedcomposite film may be formed directly on a substrate or on a lower layerfilm that is different from the above-mentioned unburned composite film.Furthermore, after the unburned composite film is formed using theabove-mentioned varnish (composition for producing a porous film), anupper layer film that is different from the above-mentioned unburnedcomposite film may be further formed in the upper layer. Note here thatin this application, both the method for providing a lower layer film onthe substrate, and the method for forming an upper layer film that isdifferent from the above-mentioned unburned composite film on thefurther upper layer by using the above-mentioned varnish after theunburned composite film is formed, are included in the method forforming an unburned composite film on the substrate. However, when theresin component contained in the above-mentioned varnish is polyamideacid or a polyamide-imide precursor, and when a material that does notneed a burning step is used for the upper layer film, an upper layerfilm may be formed on the resin-fine particle composite film afterburning may be formed. From the viewpoint that a porous film having asmooth surface whose surface roughness is small is easily formed, it ispreferable that an unburned composite film made of the above-mentionedvarnish is formed as a single layer on the substrate, or formed as anupper layer film on the lower layer film. The unburned composite filmcan be formed by, for example, applying the varnish onto the substrateor the lower layer film, and drying thereof at normal pressure or undervacuum at 0 to 100° C., and preferably at normal pressure at 10 to 100°C. Examples of the substrate include a PET film, a SUS substrate, aglass substrate, and the like.

Examples of the lower layer film (or an upper layer film) include alower (or upper) layer unburned composite film formed using a varnishfor forming a lower (or upper) layer film containing a resin includingresin including polyvinylidene fluoride, polyethersulfone, polyamideacid and/or polyimide, fine particles, and a solvent, wherein thecontent of the fine particles is more than 40% by volume and not morethan 81% by volume with respect to the total of the resin and the fineparticles. The lower layer (or upper layer) unburned composite film maybe formed on the substrate. When the content of the fine particles ismore than 40% by volume, the particles are uniformly dispersed; and whenthe content of the fine particles is not more than 81% by volume, theparticles are dispersed without causing aggregation of particles.Consequently, pores can be formed uniformly in the porous film.Furthermore, when the content of the fine particles is within theabove-mentioned range, when the lower layer (or upper layer) unburnedcomposite film is formed on the substrate, even when the substrate isnot provided with a mold release layer in advance, mold releasabilityafter film formation can be easily secured.

Note here that the fine particles to be used for the varnish for forminga lower (or upper) layer film and the fine particles to be used for thevarnish for producing a porous film may be the same as or different fromeach other. In order to increase the density of pores in the lower (orupper) layer unburned composite film, it is preferable that the fineparticles to be used for the varnish for forming the lower (or upper)layer film has a particle diameter distribution index that is equal toor smaller than that of the fine particles to be used for a varnish forproducing a porous film. Alternatively, it is preferable that the fineparticles to be used for a varnish for the lower (or upper) layer filmhas a sphericity that is equal to or smaller than that of the fineparticles to be used for the varnish for producing a porous film.

Furthermore, the average particle diameter of fine particles to be usedfor the varnish for the lower (or upper) layer film may be the same asor different from the average particle diameter of the fine particles ofthe varnish for producing a porous film. The average particle diametermay be appropriately set to be in a range from 10 to 5000 nm dependingon the application of use. In the present invention, when the varnishfor the lower (or upper) layer film is used, it is preferable that thevarnish for producing a porous film of the present invention is used forthe upper layer, and a varnish for the lower layer film having differentaverage particle diameter or a particle diameter distribution index offine particles is used in combination.

Furthermore, the content of the fine particles to be used for thevarnish for forming a lower (or upper) layer film may be larger orsmaller than that of the above-mentioned varnish. Suitable examples ofthe components such as a resin component, fine particles, and a solventincluded in the varnish for forming a lower (or upper) layer film arethe same as those in the above-mentioned varnish. The varnish forforming a lower (or upper) layer film can be prepared by the same methodas that of the above-mentioned varnish. The lower layer unburnedcomposite film can be formed by, for example, applying the varnish for alower layer film onto the substrate, followed by drying at normalpressure or under vacuum at 0 to 100° C., and preferably at normalpressure at 10 to 100° C. The same is true to the film formationconditions of the upper layer unburned composite film.

Furthermore, examples of the lower (or upper) layer film include lowerlayer films made of fiber materials such as cellulose resin, non-wovenfabric (for example, polyimide non-woven fabric or the like; a fiberdiameter is, for example, about 50 nm to about 3000 nm), and a polyimidefilm.

Furthermore, a burning step of burning the unburned composite film or alaminated film of the unburned composite film and the lower (or upper)layer film to obtain a polyimide-fine particle composite film is carriedout. When the unburned composite film or the lower layer unburnedcomposite film is formed on the substrate, burning may be carried out asit is, or the unburned composite film or the laminated film of theunburned composite film and the lower unburned composite film may bepeeled off from the substrate before carrying out the burning step.

Note here that when the above-mentioned lower (or upper) layer film in alaminated film is a lower (or upper) layer unburned composite filmformed using a varnish for forming a lower (or upper) layer film, andthe composition of the varnish for forming a lower (or upper) layer filmis the same as the composition of the varnish for producing a porousfilm to be used for formation of the above-mentioned unburned compositefilm, the laminated film of the above-mentioned unburned composite filmand the above-mentioned lower (or upper) layer film is substantially onelayer (single layer), but in this specification, it is referred to as alaminated film.

When the unburned composite film or the laminated film of the unburnedcomposite film and the lower (or upper) layer unburned composite film ispeeled off from the substrate, the substrate provided with a moldrelease layer in advance can also be used in order to further enhancethe releasability of the film. In a case of providing the substrate witha mold release layer in advance, the mold release agent is applied ontothe substrate and is dried or baked before the application of thevarnish. The mold release agent used here may be a known mold releaseagent, such as an alkylphosphate ammonium salt-based or fluorine-basedagent or silicon, without particular restrictions. When the driedunburned composite film is peeled off from the substrate, a slightamount of the mold release agent remains on the surface of the peeledunburned composite film and may lead to discoloration during burning andadverse effects on the electrical characteristics. The mold releaseagent should therefore be removed as much as possible. In order toremove the mold release agent, a washing step of washing the unburnedcomposite film or the laminated film of the unburned composite film andthe lower layer unburned composite film peeled off from the substratewith an organic solvent may be introduced.

Alternatively, when the substrate is directly used, as it is, withoutproviding a mold release layer in formation, of the unburned compositefilm or the lower layer unburned composite film, the step of forming themold release layer and the washing step can be omitted. Furthermore, inthe production of an unburned composite film, before the below-mentionedburning step, an immersion step into a water-containing solvent, apressing step, and a drying step after the immersion step may beoptionally provided.

[Production of Resin-Fine Particle Composite Film (Burning Step)]

When a resin component contained in a varnish is polyamide acid or apolyamide-imide precursor, the unburned composite film is subjected toheat treatment as post-treatment (burning) to be formed into a compositefilm (resin-fine particle composite film) composed of resin made ofpolyimide and/or polyamide-imide and fine particles. Note here that whenthe resin component contained in a varnish is polyimide, polyamide-imideor polyethersulfone, a burning step may not be carried out. In theunburned composite film forming step, when the unburned composite filmis formed on a lower layer film that is different from the unburnedcomposite film, the lower layer film together with the unburnedcomposite film is burned in the burning step. The burning temperature inthe burning step varies depending on the structures of the unburnedcomposite film and the lower layer film and the presence or absence of acondensing agent, but the temperature is preferably 120° C. to 450° C.,and more preferably 150° C. to 400° C. In a case of using an organicmaterial for the fine particles, the burning temperature need to be setto a temperature lower than the thermal decomposition temperature of theorganic material. When the resin component contained in the varnish ispolyamide acid, in the burning step, imidization is preferablycompleted.

The burning can be performed by, for example, a method of increasing thetemperature from room temperature to 400° C. over three hours and thenholding 400° C. for 20 minutes or a method of stepwise drying-thermalimidization by stepwise increasing the temperature by 50° C. from roomtemperature to 400° C. (holding the temperature of each step for 20minutes) and finally holding 400° C. for 20 minutes. When the unburnedcomposite film is formed on the substrate and the unburned compositefilm is peeled from the substrate once, an end of the unburned compositefilm may be fixed to, for example, a frame made of SUS stainless steelto prevent deformation.

The thickness of the resulting resin-fine particle composite film can bedetermined by, for example, measuring the thicknesses of a plurality ofpositions with a micrometer or the like and averaging the thicknesses.Preferred average thickness varies depending on the application of useof porous film, however, is preferably 5 to 500 μm and more preferably10 to 100 μm, and further preferably 15 to 30 μm in the use as aseparator. The average thickness is preferably 5 to 500 μm and morepreferably 10 to 300 μm, and further preferably 20 to 150 μm in the useas a filter or the like.

[Porosification of Resin-Fine Particle Composite Film (FineParticle-Removing Step)]

The porous film can be produced with high reproducibility by selectingan appropriate method for removing the fine particles from theresin-fine particle composite film.

For example, when silica is employed as the material of the fineparticles, the silica can be removed by treating the resin-fine particlecomposite film with, for example, a low-concentration hydrogen fluoridesolution to dissolve the silica.

An organic material can also be selected as the material of the fineparticles. Any organic material, which is decomposed at a temperaturelower than resin contained in the resin-fine particle composite film,may be used, without particular limitation. Examples thereof includeresin fine particles composed of linear polymers and knowndepolymerizable polymers. The linear polymer usually has a molecularchain that is randomly cleaved during thermal decomposition; and thedepolymerizable polymer is decomposed into a monomer during thermaldecomposition. Both of them are decomposed into a low molecular weightsubstance or to CO₂ and disappear from the porous film. A decompositiontemperature of the resin fine particles to be used is preferably 200° C.to 320° C. and more preferably 230° C. to 260° C. A decompositiontemperature of 200° C. or more allows formation of a film even if thevarnish contains a high boiling point solvent and broadens the selectionof burning conditions of the resin-fine particle composite film.Furthermore, a decomposition temperature of less than 320° C. allows theresin fine particles alone to disappear without thermally damaging resincontained in the resin-fine particle composite film.

The total thickness of the porous film is not particularly limited, andis preferably 5 μm to 500 μm, more preferably 10 μm to 100 μm, andfurther preferably 15 μm to 30 μm, when, for example, the porous film isused for a separator or the like. The thickness is preferably 5 μm to500 μm, more preferably 10 μm to 300 μm, and further preferably 20 μm to150 μm, when, for example, the porous film is used for a filter or thelike. Similar to the measurement of the resin-fine particle compositefilm, the above-mentioned thickness can be determined by, for example,measuring thicknesses of a plurality of positions with a micrometer orthe like and averaging the thicknesses.

When the porous film is formed of two or more types of compositions forproducing a porous film, or when it is manufactured by combination witha layer by the other composition for producing a porous film by apreparation method that is different from the manufacturing method ofthe present invention, the ratio in the thickness direction of theregion formed by each composition for producing a porous film may beappropriately determined depending on the application of use of theporous film. When a porous film has two regions, that is, a layer (I) bycomposition for producing a porous film according to the presentinvention and a layer (II) by the other composition for producing aporous film that is different from the manufacturing method according tothe present invention, the ratio ((I):(II)) of each region in thethickness direction may be adjusted to, for example, 1:99 to 99:1,preferably 5:95 to 95:5. The thickness of each layer can be calculatedby averaging thicknesses at a plurality of positions in a cross sectionof the porous film by observing under, for example, a scanning electronmicroscope (SEM).

[Resin-Removing Step]

A method for producing a porous film may include a resin-removing stepof removing at least a part of a resin portion of a resin-fine particlecomposite film before the fine particle-removing step, or removing atleast a part of the porous film after the fine particle-removing step.When at least a part of a resin portion of the resin-fine particlecomposite film is removed before the fine particle-removing step andwhen the fine particles are removed and pores are formed in thesubsequent fine particle-removing step, the opening rate and the surfacesmoothness of the porous film of the final product can be improved ascompared with the case where at least a part of the resin portion is notremoved. Furthermore, when at least a part of the porous film is removedafter the fine particle-removing step, the opening rate of the porousfilm of the final product can be improved as compared with the casewhere at least a part of the porous resin film is not removed.

The step of removing at least a part of the resin portion or the step ofremoving at least a part of the porous film can be carried out by ausual chemical etching or physical removing method, or a methodcombining these methods.

The chemical etching method includes treatment using a chemical etchantsuch as an inorganic alkaline solution or an organic alkaline solution.An inorganic alkaline solution is preferable. Examples of the inorganicalkaline solution include a hydrazine solution including hydrazinehydrate and ethylenediamine; a solution of alkaline metal hydroxide suchas potassium hydroxide, sodium hydroxide, sodium carbonate, sodiumsilicate, and sodium metasilicate; an ammonium solution; an etchantincluding alkali hydroxide, hydrazine, and1,3-dimethyl-2-imidazolidinone as a main component, or the like.Examples of the organic alkaline solution include an alkaline solutionof primary amines such as ethyl amine and n-propyl amine; secondaryamines such as diethyl amine and di-n-butylamine; tertiary amines suchas triethylamine and methyl diethyl amine; alcohol amines such asdimethyl ethanol amine and triethanolamine; quaternary ammonium saltssuch as tetramethylammonium hydroxide and tetraethylammonium hydroxide;cyclic amines such as pyrrole and piperidine, or the like.

As a solvent for each solution, pure water and alcohols can beappropriately selected. Furthermore, solvents in which an appropriateamount of surfactant is added can be used. An alkali concentration is,for example, 0.01 to 20% by mass.

Furthermore, examples of the physical method include dry etching byplasma (oxygen, argon, etc.), corona discharge, or the like, a methodfor treating a surface of a film by dispersing abrasives (for example,alumina (rigidity 9), or the like) in a liquid and irradiating thesurface of a film with the liquid at the irradiation rate of 30 to 100m/s, and the like.

The above-mentioned methods are preferable because they are applicableto the resin-removing step both before and after the fineparticle-removing step.

On the other hand, as the physical method that can be applied only tothe resin-removing step carried out after the fine particle-removingstep, a method of compression bonding a mount film (for example, apolyester film such as a PET film) whose subject surface is wetted withliquid and then peeling a porous film from the mount film after dryingor without drying can be employed. Due to the surface tension of theliquid or electrostatic adhesion, the porous film is peeled from themount film with only the surface layer of the porous film left on themount film.

<<Use of Porous Film>>

The porous film described above can be used as a separator for alithium-ion battery, an electrolyte film of a fuel cell, a separationfilm for a gas or liquid, and low dielectric constant materials. Theabove-mentioned porous film can be used as separators for secondarybatteries such as a nickel cadmium battery, nickel hydrogen battery, andlithium ion secondary battery, and particularly preferably as a porousseparator for a lithium ion secondary battery. In particular, when theporous film is used for a separator for a lithium-ion battery, in theabove-mentioned unburned composite film formation step, theabove-mentioned unburned composite film is formed on a lower layer filmthat is different from the above-mentioned unburned composite film,wherein a film formed by using the above-mentioned varnish for lowerlayer film is used as the above-mentioned lower film, and a surface atthe lower layer film side is defined as a negative electrode surfaceside of lithium-ion battery, thus improving the battery performance.

<Secondary Battery>

In a secondary battery, an electrolytic solution and a separator made ofthe above-mentioned porous film are disposed between a negativeelectrode and a positive electrode. The secondary battery may be of anytype and may have any configuration without limitation. Any secondarybatteries can be employed with no particular limitation in knownsecondary batteries such as nickel cadmium and nickel hydrogen batteriesand a lithium ion secondary battery, as long as the battery has aconfiguration in which a battery element including a positive electrode,a separator, and a negative electrode being laminated in this order soas to satisfy the above-described requirements is impregnated with anelectrolytic solution and encapsulated in an outer package.

The negative electrode of the secondary battery can have a structure inwhich a negative electrode mixture composed of a negative electrodeactive material, a conductive auxiliary agent, and a binder is formed ona current collector. For example, as the negative electrode activematerial, cadmium hydroxide can be used in nickel cadmium batteries, anda hydrogen-occlusion alloy can be used in nickel hydrogen batteries. Inlithium ion secondary batteries, a material that can beelectrochemically doped with lithium can be employed. Examples of suchactive materials include carbon materials, silicon, aluminum, tin, andWood's alloy.

Examples of the conductive auxiliary agent constituting the negativeelectrode include carbon materials such as acetylene black and Ketjenblack. The binder is composed of an organic polymer, and examplesthereof include polyvinylidene fluoride and carboxymethyl cellulose. Thecurrent collector can be, for example, copper foil, stainless steelfoil, nickel foil, or the like.

The positive electrode can have a structure in which a positiveelectrode mixture composed of a positive electrode active material, aconductive auxiliary agent, and a binder is formed on a currentcollector. For example, as the positive electrode active material,nickel hydroxide can be used in nickel cadmium batteries, and nickelhydroxide or nickel oxyhydroxide can be used in nickel hydrogenbatteries, respectively. Meanwhile, in lithium ion secondary batteries,examples of the positive electrode active material includelithium-containing transition metal oxides, specifically, LiCoO₂,LiNiO₂, LiMn_(0.5)Ni_(0.5)O₂, LiCo_(1/3)Ni_(1/3)Mn_(1/3)O₂, LiMn₂O₄,LiFePO₄, LiCo_(0.5)Ni₀₅O₂, LiAl_(0.25)Ni_(0.75)O₂. Examples of theconductive auxiliary agent include carbon materials such as acetyleneblack and Ketjen black. The binder is made of an organic polymer, andexamples thereof include polyvinylidene fluoride. The current collectorcan be, for example, aluminum foil, stainless steel foil, or titaniumfoil.

As the electrolytic solution, a potassium hydroxide solution is used in,for example, a nickel cadmium battery or a nickel hydrogen battery. Theelectrolytic solution in a lithium ion secondary battery is composed bydissolving a lithium salt in a non-aqueous solvent. Examples of thelithium salt include LiPF₆, LiBF₄, LiClO₄ and the like. Examples of thenon-aqueous solvent include propylene carbonate, ethylene carbonate,dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate,γ-butyrolactone, and vinylene carbonate. These solvents may be usedalone or as a mixture.

Examples of the outer package material include metal cans and aluminumlaminate packs. The shape of the battery is, for example, a rectangularshape, a cylindrical shape, or a coin shape; however, the separator madeof the porous film mentioned above can be suitably applied to anyshapes.

EXAMPLES

The present invention will now be more specifically described withreference to Examples, but the scope of the present invention is notlimited to the following examples.

Example 1 and Comparative Examples 1

Silica dispersion liquid (including 0.5% by mass dispersant with respectto silica) was added to a polyamide acid solution so that the amount ofpolyamide acid was 20% by mass and the mass of silica was 80% by masswith respect to the total mass of polyamide acid and silica.Furthermore, organic solvents (1) and (2) were respectively added sothat the solvent composition in the final composition became the ratioof the organic solvent (1) to the organic solvent (2) of 90:10. Theywere added in a rotation-revolution type mixer (product name: “AwatoriRentarou” manufactured by Thinky Corporation), followed by kneading at arotational speed of 2000 rpm for five minutes in Example 1 and at arotational speed of 2000 rpm for one minute in Comparative Example 1 toprepare a varnish for producing a porous film having a solid contentconcentration of 30% by mass, respectively. Note here that the ratio ofpolyamide acid to silica (polyamide acid:silica) in the obtained varnishcomposition was 28:72 in the volume ratio, and 20:80 in the mass ratio.Note here that a polyamide acid solution, an organic solvent, adispersant, and fine particles mentioned below were used.

Polyamide acid solution: reaction product of pyromellitic dianhydrideand 4,4′-diaminodiphenyl ether (solid content of 20% by mass (organicsolvent: N,N-dimethyl acetamide))

Organic solvent (1): N,N-dimethyl acetamide (DMAc)

Organic solvent (2): gamma butyrolactone

Dispersant: polyoxyethylene secondary alkyl ether dispersant

Fine particle: silica:silica having an average particle diameter of 700nm

The obtained varnish for producing a porous film was applied onto apolyethylene terephthalate (PET) film as a base material using anapplicator to form an unburned composite film. This unburned compositefilm was placed in an oven, and burned at 380° C. for 15 minutes tocomplete imidization to obtain a resin-fine particle composite film.Thereafter, the resin-fine particle composite film was peeled off fromthe base material. The resin-fine particle composite film was immersedin a hydrogen fluoride solution (HF) for 10 minutes to remove silicafine particles contained in the film, followed by washing with water anddrying to obtain polyimide porous films having a thickness of 40 μm ofExample 1 and Comparative Example 1, respectively.

The surface roughness of the obtained polyimide porous films of Example1 and Comparative Example 1 was measured by the following methods.Firstly, a porous film wetted, with water was spread over a flat glasssubstrate such that a surface of the porous film at a side that is incontact with the PET film was brought into contact with a glasssubstrate. At that time, air between the glass substrate and the porousfilm was removed using the PET film to stretch out wrinkles generated onthe porous film. Then, the porous film on the glass substrate was heatedat 70° C. for two minutes. Thus, the surface roughness of the porousfilm flatly laid over the glass substrate was measured. The surfaceroughness was measured using the stylus type surface roughness meterDektak150 manufactured by Ulvac Inc. according to the followingconditions.

Stylus radius: 12.5 μmMeasurement distance: 10000 μmMeasurement time: 120 secondsHorizontal resolution: 0.278 μm/sampleStylus pressure: 5.00 mg

As a result of the measurement using the stylus type surface roughnessmeter, the surface roughness Ra of the polyimide porous film in Example1 was 3069 Å, and the surface roughness Ra of the polyimide′porous filmin Comparative Example 1 was 30454 Å. Furthermore, results ofobservation of the surface of the porous film under a scanning electronmicroscope (SEM) are shown in FIG. 1 and FIG. 2. From FIG. 1 and themeasurement results of the surface roughness, it is shown that no stripywaviness is observed in the surface of the polyimide porous film ofExample 1, showing that the polyimide porous film of Example 1 hasexcellent surface smoothness. On the contrary, from FIG. 2 and themeasurement results of the surface roughness, it is shown that thesurface of the polyimide porous film of Comparative Example 1 had alarge height difference and stripy waviness.

Examples 2 to 4

A varnish for producing a porous film was prepared in the same manner asin Example 1 except that polyamide acid was changed intopolyethersulfone; the solid content concentration of a varnish waschanged from 30% by mass to 42% by mass; 5% by mass of phosphoric aciddispersant with respect to silica was added into a silica dispersionliquid; only DMAc was used as an organic solvent; and the ratio of resinto silica (resin:silica) in the obtained varnish was changed from 20:80to 30:70 in the mass ratio.

The obtained varnish for producing a porous film was applied onto apolyethylene terephthalate (PET) film as a base material using anapplicator to form a coating film. This coating film was baked at 50° C.for 5 minutes to form a resin-fine particle composite film. Theresin-fine particle composite film was immersed in water for threeminutes. Thereafter, the resin-fine particle composite film was peeledoff from the base material. The resin-fine particle composite film wasimmersed in hydrogen fluoride (HF) for 10 minutes to remove silica fineparticles contained in the film, followed by washing with water anddrying so as to obtain a polyethersulfone porous film of Example 2having a thickness of 40 μm. Furthermore, polyethersulfone porous filmsin the case where the baking temperature of the coating film was changedfrom 50° C. to 70° C. (Example 3) and to 90° C. (Example 4),respectively, were obtained.

The surface roughness of the obtained polyethersulfone porous film inExample 2 was measured in the same manner as in Example 1. As a resultof measurement using the stylus type surface roughness meter, thesurface roughness Ra of the polyethersulfone porous film of Example 2was 11000 Å. The surface roughness Ra values of the polyethersulfoneporous films of Examples 3 and 4 were similar values.

Example 5

A varnish for producing a porous film was prepared in the same manner asin Example 1 except that polyamide acid was changed intopolyethersulfone; the solid content concentration of the varnish waschanged from 30% by mass to 35% by mass; 5% by mass of phosphoric aciddispersant with respect to silica was added into a silica dispersionliquid; only DMAc was used as an organic solvent; and the ratio of resinto silica (resin:silica) in the obtained varnish was changed from 20:80to 30:70 in the mass ratio.

A polyethersulfone porous film of Example 5 having a thickness of 40 μmwas obtained using the obtained varnish for producing a porous film inthe same manner as in Example 2. The surface roughness of the obtainedpolyethersulfone porous film of Example 5 was measured in the samemanner as in Example 1. As a result of measurement using a stylus typesurface roughness meter, the surface roughness Ra of thepolyethersulfone porous film of Example 5 was 8000 Å.

1. A porous film of polyvinylidene fluoride, polyethersulfone, polyimide and/or polyamide-imide, wherein surface roughness Ra is 30000 Å or less.
 2. The porous film according to claim 1, wherein an opening diameter in a surface is 10 nm to 5000 nm.
 3. The porous film according to claim 1, wherein the surface roughness Ra is surface roughness Ra on a surface opposite to a surface facing a substrate during production.
 4. A porous film of polyvinylidene fluoride, polyethersulfone, polyimide and/or polyamide-imide, wherein the porous film includes a structure in which spherical pores communicate with each other, no waviness is deeper than a recess formed by an opening portion, and a diameter of the opening portion is 10 nm to 5000 nm.
 5. A method for producing a porous film of polyvinylidene fluoride, polyethersulfone, polyimide and/or polyamide-imide, the method comprising: kneading a varnish containing fine particles and resin such that surface roughness Ra of an unburned composite film containing the fine particles and the resin that is at least one resin selected from the group consisting of polyvinylidene fluoride, polyethersulfone, polyamide acid, polyimide, a polyamide-imide precursor and polyamide-imide, is 30000 Å or less.
 6. A method for producing a porous film of polyvinylidene fluoride, polyethersulfone, polyimide and/or polyamide-imide, the method comprising: kneading a varnish containing fine particles, and at least one resin selected from the group consisting of polyvinylidene fluoride, polyethersulfone, polyamide acid, polyimide, a polyamide-imide precursor and polyamide-imide, wherein the varnish has a viscosity at 25° C. of 0.1 to 3 Pa·s, a solid content concentration of 10 to 50% by mass, and an average particle diameter of the fine particles of 10 to 5000 nm, and the kneading is carried out for 2 minutes to 10 hours.
 7. The porous film according to claim 2, wherein the surface roughness Ra is surface roughness Ra on a surface opposite to a surface facing a substrate during production. 